This got started from a question on the Renderman forums about the difference between ST and UV. Renderman uses UV and ST to differiate between “implicit” and “explicit” texture coordinates. Implicit is automatically defined by the geometric shape and explicit is manually defined by the user.

With polygons and subdivision geometry, Renderman uses ST coordinates for texture mapping (it converts Maya UVs to Renderman STs behind the scenes). But it also automatically assigns UV coordinates to each face on the geometry, you might ask why would you need this?